1. Technical Field
The present invention relates to an optical head for a magneto-optical recording medium such as a photomagnetic disk, and more particularly to a focusing error detecting system for the optical head.
2. Background Art
In one known optical head, light reflected from a magneto-optical disk (hereinafter referred to as an optical disk) is separated by using a beam splitter, and an information signal and a servo signal (AF signal, TR signal) are detected according to the separated light beams from the beam splitter.
For example, FIG. 11 of the accompanying drawings shows the construction of an optical head disclosed in Japanese Patent Publication No. 62-252552. In FIG. 11, a reflected light from an optical disk 60 is reflected by a first beam splitter 61, and is then applied through a cylindrical lens 62 using an astigmatic system to a detector 63, thus providing a focusing error signal from the detector 63. A tracking signal is also obtained from the detector 63. On the other hand, the reflected light from the optical disk 60 is also reflected by a second beam splitter 64, and is then separated into two light beams by a third beam splitter 65. The separated light beams are received by respective dedicated detectors 66 and 67 to read pre-formatted data and provide an information signal according to the sum and the difference between detection signals from the detectors 66 and 67.
Another type of optical head using an optical fiber is disclosed in Japanese Patent Publication No. 59-56239 for example. In this prior optical head, a reflected light from an optical disk is separated by using a first beam splitter, and a separated beam from the first beam splitter is further separated into a transmitted light beam and a reflected light beam by a second beam splitter. These light beams from the second beam splitter enter respective optical fibers, and are then emitted from the other ends of the optical fibers to respective detectors.
More specifically, as shown in FIG. 12, reflected light from an optical disk is reflected by a first beam splitter 71, and is then converged by a converging lens 72. The converged light is separated into a transmitted light beam and a reflected light beam by a second beam splitter 73. These light beams are individually applied to a first optical fiber 74 and a second optical fiber 75. The second beam splitter 73 has different sizes in its transmitting direction and reflecting direction. Further, the first optical fiber 74 and the second optical fiber 75 have respective inlet ends disposed before and behind a focal point of the converging lens 72. The light beams emitted from the first and second optical fibers 74 and 75 are made into parallel beams by collimate lenses 76 and 77, respectively, and each of the parallel beams is then divided into two beans by polarizing beam splitters 78 and 79. The divided beams are individually received by four detectors 80, 81, 82 and 83, thus obtaining a focusing error signal by computing outputs from the detectors 80-83.
However, in the conventional system shown in FIG. 11, the reflected light from the optical disk is separated by the first and second beam splitters 61 and 64, and the separated light beams are used for obtaining the servo signal and the information signal. As a result, it is difficult to make the optical head compact. Furthermore, since the focusing error signal and the tracking signal are detected by the same detector 63, there is a possibility that the tracking signal will leak into the focusing error signal.
In the conventional system shown in FIG. 12, the path of the light reflected from the optical disk is changed path by using the first beam splitter 71, and is then applied to the inlet ends of the optical fibers 74 and 75 by using the second beam splitter 73 which has different sizes in its transmitting direction and reflecting direction. Accordingly, when the position of the second beam splitter 73 slips the quantity of light entering the optical fibers 74 and 75 is greatly changed, resulting in the easy generation of an error in the focusing error signal. Furthermore, adjustment for introducing the reflected light from the optical disk to the inlet ends of the optical fibers 74 and 75 is critical and difficult.
Further, while the optical head shown in FIG. 12 is separated into a fixed portion and a movable portion to make the movable portion compact and light, the movable portion includes two beam splitters, two lenses and three optical fibers inclusive of an optical system from a laser light source 84. Thus, the construction of the movable portion is not sufficiently compact.